1. Field of the Invention
This invention pertains generally to gas separation apparatus using an electric field. More specifically, the present invention uses non-liquid cleaning techniques to maintain electrostatic precipitator electrodes. In a most specific manifestation, a new method and apparatus are provided to dislodge ash from collection plates within an electrostatic precipitator.
2. Description of the Related Art
Industries as diverse as mills, pharmaceutical or chemical, food processing, and cement kilns must separate contaminants or particulates from an air or gaseous stream. The gases may be a product of combustion, such as present in an exhaust stack, but may also represent other gas streams and may contain such diverse materials as liquid particulates, smoke or dust from various sources, and the like. Separators that must process relatively large volumes of gas are common in power generating facilities and factories.
The techniques used for purification of gas streams have been diverse, including such techniques as filtration, washing, flocculation, centrifugation, and electrostatic precipitation. The techniques have heretofore been associated with certain advantages and disadvantages; hence have limited application.
In filtration, particulates are separated through a mechanical filter which selectively traps particles of a minimum size and larger. Unfortunately, flow through a filter is limited by the surface area and cleanliness of the filter. The filter material must be both durable and simultaneously open and porous. In higher volume systems, and in corrosive or extreme environments, filters tend to clog quickly and unpredictably, and present undesirable resistance to the passage of the gas stream. During the period of filter changing or cleaning, which can be particularly tedious, the machine, equipment, or process must be stopped or diverted. This shut-down requires either a duplicate filtration pathway, which may add substantial cost, or a shut-down of the machine or process. Until recently, these limitations presented design challenges that have primarily limited this technology to low volume purification.
Washing offers an advantage over dry filtration in presenting the opportunity for selective gas or liquid particulate separation and neutralization, and in reduced gas flow resistance. Unfortunately, the liquid must also be processed; and where there are high levels of particulates, the particulates must be separated from the liquid by yet another process, or the liquid and particulates must be transported to some further industrial or commercial process or disposal location. The added weight and difficulty of handling a liquid (in addition to the particulate) during transport makes liquid separation less desirable in many instances, particularly where there may be a demonstrated application for the particulate content within the gas stream.
Similar to washing, flocculation necessitates the introduction of additional materials that add bulk to the waste stream and unnecessarily complicate the handling and disposal of the contaminants. Furthermore, the flocculating materials must also be provided as raw materials, which may add substantial expense in the operation of such a device. Consequently, flocculation is normally reserved for systems and operations where other techniques have been unsuccessful, or where a particular material is to be removed from the gas stream which is susceptible to specific flocculent that may provide other benefit.
Centrifugation presents opportunity for larger particle removal, such as separation of sand or grit from an air stream. However, centrifugation becomes slower and more complex as the size of the entrained particles or liquids become smaller. Consequently, in applications such as the removal of fly ash from a combustion stream, centrifugation tends to be selective only to relatively large particles, thereby leaving an undesirably large quantity of fine fly-ash in the effluent stream.
Electrostatic precipitators have demonstrated exceptional benefit for contaminants including fly ash, while avoiding the limitations of other processes. For example, unlike centrifugation, electrostatic precipitators tend to be highly effective at removing particulates of very minute size from a gas stream. The process provides little if any flow restriction, and yet substantial quantities of contaminants may be removed from the air stream.
When contaminants pass through an electrostatic precipitator, they pass between discharge electrodes and collection electrodes, which transfer an electrostatic charge to the contaminants. Once charged, the contaminants will be directed by the charge force towards the oppositely charged collecting electrodes. The collecting electrodes are frequently in the form of plates having large surface area and relatively small gap between collector plates. The dimensions of the plates and the inter-electrode spacing is a function of the composition of the gas stream, electrode potential, particulate size of contaminants, anticipated gas breakdown potential, and similar known factors. The selection of dimension and voltage will be made with the goal of gas stream purification in mind, and in gas streams where very fine particulate matter is to be removed, such as with fly ash, relatively high voltage potentials and larger plates may be provided. The proper transfer of charge to the particulates and the subsequent electrostatic attraction to collector plates is vital for proper operation.
By design, the collector plates will accumulate contaminants. As electrically non-conductive particles are deposited, the layers of accumulating particles develop an electrical potential gradient through the thickness of the deposited layer, whereby the voltage at the exposed surface decreases in electrical potential, and possibly even reverses charge. When a sufficiently thick layer of electrically non-conductive particles has accumulated to reduce the surface potential, further significant particulate capture becomes difficult or impossible. Consequently, and in spite of the many benefits, electrostatic precipitators have heretofore been limited in efficiency by the effects of the contaminants on the collection plates.
In order to provide continuous efficient operation of the precipitator, a number of automatically controlled cleaning techniques are used. One almost universal technique used in dry electrostatic precipitators is the use of a mechanical rapper device. The rapper creates vibration in the collector electrodes, in turn causing the precipitate to drop off of the electrodes. Generally the precipitate drops under the influence of gravity or is carried by a special air stream into a separate container for final disposal.
Several patents are exemplary of the use of rappers, including Brandt in U.S. Pat. No. 3,274,753; Johnston et al in U.S. Pat. No. 5,173,867, Lund in U.S. Pat. No. 5,792,240; and Terai et al in U.S. Pat. No. 6,336,961, each of which is incorporated herein by reference for their teachings of rapper systems for use with electrostatic precipitators. Unfortunately, the mechanical rapper systems of the prior art have been known to require substantial cycle times, and the mechanical forces tend to move the contaminant back into the gas stream. Furthermore, rapper systems tend to be maintenance intensive, and, for high resistivity particulate, the rapper tends to be relatively ineffective, owing to the accumulation of electrical charge on the particulate surface.
Since the release of undesirable contaminants entrained within the gas stream is undesirable, other techniques besides mechanical rappers have been proposed. Gallo et al in U.S. Pat. No. 5,378,978 and Shevalenko et al in U.S. Pat. No. 4,536,698 each illustrate electronic systems to control the accumulation of precipitate upon the electrodes. In particular, the control system of Gallo et al illustrates the challenges of prior art systems, including many components and much complexity. What is desired then is a method or apparatus to overcome these limitations of the present electrostatic precipitators.